1. Field of the Invention
The invention relates generally to methods of forming wood composite panels having an embossed front face and, more particularly, the invention relates to methods of die design for the embossing of the back faces of such panels to effect substantially uniform, acceptable product performance when more than one front face pattern is being embossed onto a panel or panels by a single press during a press cycle. The invention also includes methods of forming wood composite panels using dies made according to the inventive die design methods.
2. Description of Related Technology
It is often desirable to produce wood composite panels having an embossed or molded front face in applications where the appearance of the wood is important, such as for simulated wood siding. The presses used for compressing raw material wood composite mats into consolidated, integral panels may be large and adapted to hold multiple sets of embossing dies. Thus, it may be desirable to produce numerous wood composite panels during a press cycle of a single press with each panel having a different pattern embossed thereon. Furthermore, it may be desirable to impress more than one pattern onto a single panel.
However, problems arise when an attempt is made to simultaneously consolidate and emboss a panel or panels with more than one embossing pattern. As discussed more fully below, because the press is not equipped to provide individual caliper control at various locations along a raw material mat where an increase or decrease in pressure may be needed to adequately compress the mat, die set designs including embossed upper dies and flat lower dies often do not produce panels with acceptable characteristics such as sufficient physical strength. In order to further explain this problem, the formation of a conventional flat panel shall first be discussed.
A flat panel is produced by compressing a mat of raw composite material between an upper die and a lower die attached to respective upper and lower heated platens of the press. Am at of raw material may be prepared by a variety of processes and comprise a variety of shapes or types of raw wood fragments such as wood flakes, strands, particles, chips and fibers. In what is generally referred to as a dry process, the raw wood fragments are pre-coated with a thermosetting resin binder and remain generally disassociated until pressed with heat which cures the resin binder. The mat may also be produced by a wet process wherein (1) cellulosic fibers are blended with water to form a slurry and (2) most of the water is removed to form a wet mat of cellulosic material.
The press and attached die set provide a substantially uniform pressing force across the surface of the mat being pressed. In other words, the sum of the contact forces over the entire face of a mat, which is equal to the total force exerted by the mat on a die, is equal at each location or point on the mat. As a result, each mat placed within a press during a single press cycle ideally compresses to the same nominal thickness.
A panel with an "ideal" nominal thickness is made from a pressing process utilizing reasonable temperature, pressure and time values and the least amount of raw material possible to produce a panel exhibiting adequate strength and other desired characteristics. If a panel is pressed beyond its "ideal" nominal thickness, overdensification occurs resulting in undesirable characteristics such as brittleness and blistering.
The design criterion of an "ideal" nominal thickness for a flat panel has also been used for producing panels having an embossed surface. It has been found however that when a mat is pressed between a first die having an embossing surface and a second die having a generally flat surface, an increased surface pressure is required to compress the mat where the resulting product is thinner than the "ideal" nominal thickness. Thus, the remainder of the mat does not compress to the "ideal" nominal thickness. Stated in another way, the most compressed portion of a mat may act as a stop, barring the adequate compression of the remainder of the mat or mats being compressed at the same time. Likewise where an embossing die includes cavities resulting in mat portions not being pressed to the "ideal" nominal thickness, the remainder of the mat (or other mats being simultaneously pressed) may compress to a thickness which is less (thinner) than the "ideal" nominal thickness.
Where a single decorative pattern is embossed onto a panel or panels, the above-described thickness variations are not problematic because the generally flat, non-embossing lower die may be replaced with an embossing die specifically designed to cooperate with the upper die to form a panel having acceptable thickness and density ranges. For example, the contour of the lower die may be designed to roughly complement the contour of the decorative pattern on the upper die. In this manner, the upper and lower embossing dies become a generally "mating" die set similar to what is used in the molding art. Examples of methods of preparing wood composite panels using cooperating upper and lower dies are described in Wheeler, U.S. Pat. No. 3,868,300 (Feb. 25, 1975) and Schultz, U.S. Pat. No. 4,726,881 (Feb. 23, 1988).
However, when more than one embossing pattern is pressed onto a mat or when multiple mats are pressed with different patterns during a single press cycle, merely providing "mating" first and second dies does not produce panels with acceptable thickness and density ranges. Because of the pattern variations, overdensification or underdensification of certain portions of the panels typically results. Laboratory-scale tests used to simulate conditions found in large multi-opening presses have shown as much as a seventeen percent difference between desired and obtained thicknesses for embossed products formed during the tests. Therefore, such panels have not performed adequately when tested for properties such as adequate physical strength and uniform, minimal moisture absorption.
One solution to this problem is to use more raw material in the press (ie. raw material mats of greater density). This would increase the overall nominal pressing thickness of the final product. Therefore, the thickness differences in an embossed panel would be a smaller percentage of the overall thickness and the range of densities occurring within a panel would be reduced. However, this solution is unacceptable because of the increased cost associated with the use of additional raw material.
Another solution would be to utilize press/die set combinations adapted to provide individual caliper control at each die set. In large presses which, for example, may include as many as fourteen openings for the insertion of raw material mats (i.e. fifteen horizontal, parallel platens, thirteen of which have both an upper and a lower die mounted thereon), the addition of a means for precise individual caliper control would be complex and therefore expensive.